Wednesday, December 26, 2012

In our last post, we looked at how aberrations are the root cause
of the use of multiple lenses in constructing a complete camera lens.
Today, and over the course of future posts, let us consider in more
detail the concept of aberration.

Most generally, aberration
refers to the distortion of an image when passed through an optical
system, such as digital camera lenses. Limitations of the system might
lead to blurring. A common misconception about aberrations, however, is
that they are caused by imperfections in the device or system. Rather,
the real root cause of aberrations is in fact in the relatively
simplistic and, therefore somewhat inaccurate, boundaries set by
paraxial theory--that theory around which optical systems are built. To
put it more succinctly, even lenses built perfectly to the
specifications of paraxial theory will experience aberrations because
the theory does not perfectly describe the behavior of light in such
systems.

Aberrations can be broken down into two major
categories. Those derived from geometrical structure and those derived
from the effects on wavelength due to dispersion. Though shape and
material are important to both categories, the shape will obviously play
a larger role in the former, causing blurring, and material/density
will play a larger role in the latter, causing shifts in coloration in
addition to blurring. Geometrically derived aberrations are called
monochromatic because they affect even monochromatic light, whereas
dispersion-based aberrations are called chromatic because they are only
evident in situations with multiple color variants.

Digital camera lenses correct for these aberrations and multiple subcategories
therein. Soon, we will take a look at some of the ideas underlying the
most common chromatic and monochromatic aberrations.

Monday, December 17, 2012

The basic principle behind a lens is simple: the device mimics the eye, capturing and ushering light toward a processing center, one that is either immediate (digital) or delayed (chemical and film). Yet, the actual physical properties of light and the shape of a lens make the situation slightly more complicated.

First off, a camera lens worth its salt is rarely in fact just one lens. We call it “a lens” out of habit, but in most cases it is in fact a conglomeration of many lenses, referred to most commonly as lens elements. “Lens elements” as a phrase is itself misleading. In fact, the double meaning of the term "lens," referring to a camera part, and "optical lens," referring to any lens used for a variety of purposes, lends itself to a kind of linguistic and lexical problem: when referring to the parts that make up the whole, both have the same name. Suffice it to say that most camera lenses are not lenses at all in the most traditional sense, but rather are themselves a series of optical lenses collected together to make a more complex and precise device.

Why are there multiple lenses in digital camera lenses? The simple answer is that a single lens has what is termed “aberrations.” These are areas of the lens where light becomes distorted, either losing focus or distorting coloration, and in fact a host of other issues. The multiple lenses serve to correct these aberrations.